diff --git a/hggit/git_handler.py b/hggit/git_handler.py
index bb0824a1f7c416693ecc59cd5c32e6e6f7d2f5cf_aGdnaXQvZ2l0X2hhbmRsZXIucHk=..988d84cc5565183319a50746323129e637fd4759_aGdnaXQvZ2l0X2hhbmRsZXIucHk= 100644
--- a/hggit/git_handler.py
+++ b/hggit/git_handler.py
@@ -1097,6 +1097,8 @@
         graphwalker = self.git.get_graph_walker(heads=haveheads)
 
         def determine_wants(refs):
+            if refs is None:
+                return None
             filteredrefs = self.filter_refs(refs, heads)
             return [x for x in filteredrefs.itervalues() if x not in self.git]